1. Field of the Invention
The present invention relates to a novel lighting circuit for a vehicular discharge lamp. More particularly, this invention pertains to a novel lighting circuit for a vehicular discharge lamp designed to shorten the time needed for the flux of light from a discharge lamp to become stable after energizing the discharge lamp.
2. Description of the Related Art
Compact metal halide lamps have recently been receiving greater attention as a light source for vehicles. Because of slow starting of such metal halide lamps, in order to shorten the startup time, it has been proposed to feed an excessive lamp current several times greater than the steady state lamp current to a discharge lamp immediately after its energization to rapidly warm up the glass bulb, thereby accelerating the lighting action.
Referring to FIG. 1 a curve a indicated by a dash and dotted line shows an example of control concerning a lamp voltage (V.sub.L) and a lamp current (I.sub.L) in the case where the lighting of the discharge lamp starts with the glass bulb of the lamp being cold (this event will be hereinafter called "cold starting").
As is apparent from the curve a, when the lamp voltage V.sub.L is low, an excessive current (its effective value is denoted by "I.sub.o ") flows in an area A reaching a point m and the effective value of the current becomes I.sub.c in an area following the area A after transition from the point m to a point m'.
In this case, with a metal halide lamp with the rated power of 35 W in use as an example, I.sub.o is approximately 5 to 10 times I.sub.c.
Incidentally, the hyperbola denoted by "Pm" in FIG. 1 is a steady power line passing the point m.
If an excessive current is applied to the discharge lamp at the beginning of the lighting, the flux of light from the lamp indeed rises very sharply, but applying excessive power to the lamp increases the overshoot or undershoot, thus requiring time for the flux of light from the lamp to become a stable rated level.
A curve b in FIG. 2 indicated by a broken line schematically represents a time-dependent change in flux of lamp light (denoted by "L") corresponding to the V.sub.L -I.sub.L control shown by the curve a in FIG. 1 (the time being represented by "t"). The curve b sharply rises from t=0 (the beginning of the lamp lighting taken as an origin) to L.sub.m, a peak of the flux of light, has an overshoot o and an undershoot u, and becomes stable later at L.sub.c, the rated flux of light.
Here, the "overshoot" is defined as the amount of the flux of light greater than the rated flux of light L.sub.c as a reference, while the "undershoot" is defined as the amount of the flux of light below L.sub.c. Further, the time for the flux of light to become stable or "flux stabilizing time" is defined as the time for the flux of light L to converge within L.sub.c .+-..alpha. (where .alpha. is a value that defines the practical allowable range of the rated flux of light), and the flux stabilizing time for the curve a is denoted by "t.sub.m ".
As apparent from FIG. 2, when the above-described method of increasing power supplied to the lamp at the beginning of the lamp lighting to shorten the flux stabilizing time is utilized, the overshoot o becomes an excessive value (and at the same time wear-out of the electrodes becomes prominent) when the supplied power is too large. When transition to a steady power control area through the control area A where large power is supplied at the beginning of the lamp lighting is not properly performed, a large undershoot u will occur, making the flux stabilizing time undesirably longer.